1. Technical Field
The present invention relates to a semiconductor device and a method for manufacturing the same.
2. Background
In DRAM (Dynamic Random Access Memory) units, defective components (i.e., the transistor and the capacitor) can cause charge to leak into or from capacitors and destroy the information inside the memory cells. To avoid losing information, the DRAM memory cells need to be frequently refreshed.
The leakage of charge may occur from a capacitor to the substrate, or through the access transistor. The latter phenomenon, referred to as “sub-threshold leakage,” occurs because the channel of the field effect access transistor can never be rendered entirely non-conducting by the gate voltage. Charge leakage is a long-term issue and needs an effective solution.
In addition, a contact plug is used to connect a source/drain region to a component such as a capacitor or a digit line in a DRAM. The contact plug may be formed with metal or doped SEG (selective epitaxial growth) silicon material as shown in U.S. Pat. No. 6,399,488 B2. The SEG silicon contact plug in U.S. Pat. No. 6,399,488 B2 has two portions. The first portion is formed by simultaneously performing a selective epitaxial growth process and a thermally doping process. The second portion is formed by performing a selective epitaxial growth process until the second portion is completely formed, and then a doping process is performed. Phosphorus is used as the doping impurity. However, if phosphorus diffuses from the contact plug to the source/drain regions, the doping concentration of the source/drain regions may be increased. Moreover, the growth of the SEG silicon layer is difficult to control. If the SEG silicon layer is formed too thin, the uniformity of the SEG silicon layer is poor; if the SEG silicon layer is formed too thick, two adjacent SEG contact plugs may connect to each other.